Polarity of the metal-carbon bond

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

The compounds of lithium and magnesium are the most important of the group IA and IIA organometallics from a synthetic perspective. The metals in these two groups are the most electropositive of the elements. The polarity of the metal-carbon bond is such as to place high electron density on carbon. This electronic distribution is responsible for the strong nucleophilicity and basicity of these compounds. 

Three mechanisms can be proposed for the intimate reaction mechanism for c-e, analogous to the organic 2+2 cycloadditions a pericyclic (concerted) mechanism, a diradical mechanism, and a diion mechanism. In view of the polarization of the metal(+) carbon(-) bond, an ionic intermediate maybe expected. The retention of stereochemistry, if sometimes only temporary, points to a concerted mechanism. 

It is, nevertheless, not possible to classify organometallic compounds strictly into different types such as homopolar and heteropolar, since the physical and chemical properties of these compounds alter continuously within a given Period or Group. In a given main Group the polarity of the metal-carbon bond and thus the salt-like character of the compounds increase slowly from top to bottom, and in a given Period from left to right. 

The degree of polarity of the metal-carbon bond has, however, a marked effect, not only on the physical, but also on the chemical behavior of an organometallic compound. Thus the reactivity is found to be greater the more polar is the metal-carbon bond. This depends mainly on the difference in electronegativity between the central atom and carbon the smaller the Pauling electronegativity of a metal the greater is the reactivity of its carbon derivatives. 

Similar remarks apply to the sensitivity of organometallic compounds to oxygen. The polarity of the metal-carbon bond certainly plays an important... 

The first step in decomposition would normally be nucleophilic attack of the lone electron pair of the water oxygen atom on an empty orbital on the organometal. It is therefore connected with the existence of low-lying empty orbitals on the organometal. Such attack is related to the polarity of the metal carbon bond, strongly polarized species being... 

The polarization of the metal-carbon bonds in the indicated direction may also be inferred from direct experimental evidence. Nearly all alkyl transition metal compounds are cleaved by protonic solvents or acids, and in all cases the proton ends up with the alkyl group giving an alkane, whereas the anionic part of the cleaving agent adds to the metal A few examples are given below ... 

The usual representation of Schrock-type nucleophilic carbenes as electron rich at carbon can be especially misleading in the case of the Tebbe reagent and related complexes. These high oxidation state complexes are electron-deficient and electrophilic at the metal center, and it is unlikely for polarization of the metal-carbon bond to remove even more electron density from the metal under these circumstances. Thus, the reactivity of the Tebbe reagent is more closely related to the electrophilicity and oxophilicity of the metal center than to the nucleophilicity of a polarized carbene carbon that is, the reactivity is due to carbonyl polarization upon complexafion, not attack of the alkylidene carbon on an unactivated, electrophilic carbonyl carbon. 

The trend can be explained as follows the electron-releasing alkyl groups cause a stronger polarization of the metal-carbon bond, but more electronegative electron-withdrawing ligands lower the reaction rate. This a effect has been confirmed by model calculations [1]. 

According to Rochow et al [Ro 57,67], as a result of the interaction between an organometallic compound and a donor solvent (coordination of the donor atom of the solvent to the metal) the polarity of the metal->carbon bond in the organometallic compound decreases, causing a decrease in the reactivity of the compound. Pauson [Pa 68] states that the reactivity of the organometallic compound decreases in proportion to the increase in the donor strength of the solvent. 

The opposite view is held by Okhlobystin [Ok 67], according to whom complex formation causes an increase in the polarity of the metal-carbon bond, which must... 

Not only are the organo derivatives of Al Ga and In better acceptors than their boron analogues but they also are much more reactive towards cleavage by protonic acids, probably on account of the greater polarity of the metal-carbon bonds. This has already been noted in connection with their easy hydrolysis. Adducts with primary and secondary amines eliminate methane on heating. 

The polarity of the metal-carbon bond increases upon going down in the periodic table the lithium alkyls have some covalent character and form tetrameric clusters, whereas cesium alkyls are purely ionic. The degree of clusterification of lithium alkyls varies with the nature of the solvent between dimer (LiCHs TMEDA) and hexamer (Li-n-C4H9 cyclohexane), as can be checked by osmometry, NMR and EPR. Li NMR (I =3/2 abundance 92.6%) and Li NMR (I = 1/2 abundance 7.4%) allow to also show the dynamic fluxionahty phenomena around the Li4 tetrahedron, reversible dissociation of tetramers to dimers and ion pairs (contact ion... 

The electronegativity of divalent metals of the groups 2 (alkaline earth Be, Mg, Ca, Sr, Ba) and 12 (Zn, Cd, Hg) is between 0.9 for the more electropositive one (Ba) and 2.0 for the less electropositive one (Hg), Mg being intermediate with 1.3. The electropositivity regularly decreases as follows Ba > Sr > Ca > Mg > Be > Zn > Cd. The polarity of the metal-carbon bond follows the same trend, of course. The metals of group 12 are more electron-rich because of the presence of the full d electron shell (these electrons are not involved in the reactions). 

Donor solvents generally reduce molecular association between the metal and the carbon atom of the alkyl groups. The increased electron density around the metal atom, due to coordination with the solvent molecules with Lewis base nature like ethers and amines, enhances the polarization of the metal—carbon bond. Due to this, the carbanionic character of the organic groups gets enhanced. 

Most of the classes of organometallic compounds which have found extensive use as synthetic reagents are derivatives of the elements considered in this chapter. In most cases their synthetic value is due to the polarity of the metal-carbon bond, e.g. the reactivity of the lithium alkyls and Grignard reagents is largely due to the polar Li(S-l-)—C(8—) and Mg(8-t-)—C(8—) bonds which very readily react with polar (or in some instances particularly polarizable) groups ... 

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